The present invention relates generally to a high dielectric-constant dielectric ceramic composition and its fabrication process, and more particularly to a high dielectric-constant dielectric ceramic composition, which is suitable for multilayered ceramic capacitors, and has little, if any, capacitance change and dielectric loss over a wide temperature range (-55.degree. C. to +150.degree. C.) and its fabrication process.
Currently available capacitors, for the most part, are multilayered capacitors that are obtained by the thermocompression bonding and co-firing of a stack comprising alternately interleaved starting dielectric sheets and electrodes. These capacitors, albeit being reduced in size, achieve larger capacities than would be possible with conventional simple plate type capacitors. To achieve size reductions and capacity increases, it is essentially required to use high dielectric-constant materials and make dielectric layers thin.
So far, dielectric ceramic compositions comprising barium titanate (BaTiO.sub.3), and a bismuth compound such as Bi.sub.2 O.sub.3.2SnO.sub.2 or Bi.sub.2 O.sub.3.2ZrO.sub.2, and Ta.sub.2 O.sub.5, Sm.sub.2 O.sub.3, Nb.sub.2 O.sub.5 or the like added thereto have been used as those having a high dielectric constant and showing a reduced rate of dielectric constant change with temperature.
When the dielectric ceramic compositions comprising such ingredients have an increased dielectric constant, however, they cannot be practically used because of an increase in the rate of capacitance change with temperature. With these compositions, it is thus difficult to obtain a multilayered ceramic capacitor of small size yet of large capacity. A capacitor, even when it is somehow obtained with such compositions, barely satisfies the X7R standard (providing that the rate of capacitance change is within .+-.15% of a reference value at 25.degree. C. over the temperature range of -55.degree. C. and +125.degree. C.) prescribed by EIAJ (the Standards of Electronic Industries Association of Japan), and has some considerable dielectric loss (tan .delta.) as well. Thus, the aforesaid dielectric ceramic compositions are unsuitable for multilayered ceramic capacitors.
A problem with the incorporation of a bismuth compound in a dielectric ceramic composition is that the bismuth ingredient evaporates off upon firing, resulting in the bending of the ceramic composition material or the generation of pinholes therein. This makes it difficult to obtain a dense ceramic composition.
When a multilayered capacitor is built up of barium titanate containing a bismuth compound, palladium or a silver-palladium alloy forming an internal electrode reacts with bismuth that is one component of the dielectric material, and so the internal electrode loses its own function. For this reason, costly noble metals such as platinum, and platinum-palladium alloys, which are unlikely to react with bismuth, must be used for the internal electrode. This is one leading cause for the cost rise of multilayered ceramic capacitors.
To provide a solution to such problems as mentioned above, bismuth compound-free dielectric ceramic compositions with a high dielectric constant have already been disclosed. Some of these compositions are found to be lower in the rate of capacitance change with temperature than bismuth compound-containing compositions.
For instance, JP-A's 4-292458, 4-292459 and 4-295048 disclose high dielectric-constant dielectric ceramic compositions, which contain as major components 94.0 to 99.0 mol % of BaTiO.sub.3, 0.5 to 3.0 mol % of Nb.sub.2 O.sub.5 and 0.5 to 3.0 mol % of CoO, and as a subordinate additive 0.2 to 7.0% by weight of at least one of BaTiO.sub.3, SrZrO.sub.3 and BaZrO.sub.3. However, these compositions fail to satisfy the X8R standard (providing that the rate of capacitance change is within .+-.15% of a reference value at 25.degree. C. over the temperature range of -55.degree. C. to +150.degree. C.) prescribed by EIAJ (the Standards of Electronic Industries Association of Japan). While some compositions may somehow satisfy this standard, they are still unsuitable for multilayered ceramic capacitors because of their large dielectric loss (tan .delta.) . Upon formed into multilayered ceramic capacitors, they satisfy the X8R standard only in a very narrow temperature range of 1,280 to 1,320.degree. C. because their rates of capacitance change with temperature is largely dependent on firing temperature.
JP-A 5-109319 discloses high dielectric-constant dielectric ceramic compositions, which contain as major components 94.0 to 99.0 mol % of BaTiO.sub.3, 0.5 to 3.0 mol % of Ta.sub.2 O.sub.5 and 0.5 to 3.0 mol % of ZnO, and as a subordinate additive 0.2 to 7.0% by weight of CaZrO.sub.3. However, these compositions have a reduced insulating property, and a low relative dielectric constant as well. In addition, they fail to satisfy the X8R standard prescribed by EIAJ. Some of the compositions may possibly meet this standard, but they are still unsuitable for multilayered ceramic capacitors because their dielectric loss (tan .delta.) is unacceptably large. Furthermore, they must be fired at an elevated temperature for the formation of a multilayered ceramic capacitor, and are poor in sinterability as well.
JP-A 6-243721 discloses high dielectric-constant dielectric ceramic compositions, which contain as major components 94.0 to 99.0 mol % of BaTiO.sub.3, 0.5 to 3.0 mol % of Nb.sub.2 O.sub.5 and 0.5 to 3.0 mol % of ZnO, and as a subordinate additive 0.2 to 7.0% by weight of at least one of CaZrO.sub.3, SrZrO.sub.3 and BaZrO.sub.3. However, these compositions again fail to meet the X8R standard prescribed by EIAJ. Some of them may possibly satisfy this standard, but they are still unsuitable for multilayered ceramic capacitors because their dielectric loss (tan .delta.) is unacceptably large. Upon formed into multilayered ceramic capacitors, they may somehow satisfy the X8R standard only in a very narrow temperature range of 1,280 to 1,320.degree. C. because their rates of capacitance change with temperature is largely dependent on firing temperature, as will be appreciated from the examples given later.
Never until now, as mentioned above, is there obtained a high dielectric-constant dielectric ceramic composition, which is suitable for a multilayered ceramic capacitor and has little, if any, capacitance change and dielectric loss over a wide temperature range of -55.degree. C. to +150.degree. C.
Thus, it is an object of the present invention to provide a high dielectric-constant dielectric ceramic composition, which is most unlikely to delaminate and so suitable for a multilayered ceramic capacitor, and has little, if any, capacitance change and dielectric loss over a wide temperature range of -55.degree. C. to +150.degree. C.